A sauce is separating in the can. The obvious answer is to build more body into it, and body is easy to build. The sample looks right, the separation is gone, and the change goes forward as a formulation improvement.
It is not only a formulation change. Viscosity is one of the few things a formulator controls that changes how heat gets into the container, and heat getting into the container is what decides how long the product spends in the retort. A thicker fill can mean a longer process, and a longer process means everything in the can receives more heat than it did before, including the parts that were already receiving too much.
Two ways heat gets into a can
Heat has to travel from the retort into the coldest part of the contents. There are two ways it does that, and which one applies is decided largely by how thin the product is.
Convection. A thin, fluid product moves while it heats. Liquid near the hot wall rises, cooler liquid falls to take its place, and the whole contents circulate. The heat is carried by the movement, so it spreads quickly and fairly evenly. A brine, a thin sauce or a liquid pack behaves this way.
Conduction. A thick or solid product does not move. Heat has to pass from molecule to molecule from the wall inwards, which is slow, and it gets slower the further it has to go. A paste, a thick sauce or a solid pack behaves this way.
The difference between the two is not small. A can heating by convection reaches a given internal temperature far sooner than an identical can heating by conduction, because in one case the heat is being carried and in the other it is being handed along.
The cold spot decides everything
Inside every can there is a point that heats last. The whole process is scheduled around it, because if that point has received enough heat then so has everything else.
Where it sits depends on how the can is heating. In a convection pack the contents are circulating, so the slowest point tends to sit low in the can rather than at its geometric middle. In a uniform conduction-heated pack it is often near the geometric centre; the actual location must be established for the product, container and process.
Changing viscosity can alter both the heating rate and the location of the slowest-heating region. Both must be considered when the process is evaluated.
Convection and conduction, and where the cold spot sits
The curve can break in the middle
There is a case that catches people out, and it is directly a formulation matter.
Some products start out fluid and thicken during heating. A starch that has not yet gelatinised is thin, so the can begins by heating fast, by convection. Then the starch reaches its gelatinisation range, viscosity climbs sharply, the circulation stops, and the same can carries on heating by conduction instead.
The heating curve breaks in the middle. The first part of the process is fast and the second part is slow, and a product that was assumed to heat one way is heating both ways in sequence.
Which starch is used, how much of it, and at what temperature it swells therefore change the shape of the heating curve, not merely the texture of the finished product. This is the clearest case anywhere in the category where an ingredient decision and a process decision are the same decision.
What else slows it down
Viscosity is the largest lever but not the only one.
Particulates. Solid pieces in a liquid interrupt circulation and add conduction paths through each piece. A can of vegetables in brine and a can of the same vegetables in a thick sauce heat very differently.
Fill weight and headspace. In an agitating or rotary retort the contents are moved mechanically, which forces convection even in products that would not circulate on their own. How well that works depends on headspace and on viscosity, so a thicker product loses some of the benefit of the machine it is being processed in.
Container size and shape. The larger the can, the further heat has to travel to the middle, and the penalty for slow heating grows with it.
And the cost is paid by everything else
Here is why this matters beyond the schedule.
The cold spot has to receive enough heat. Everything else in the can receives more than that, and the slower the heating, the wider the gap between the coldest part and the rest. Extending the process to satisfy the centre means the contents near the wall spend longer at temperature than they already were.
That is where the quality goes. Softening, colour darkening, loss of the lighter flavour notes and continued water loss from solids all keep accumulating for as long as the product is hot. A change made to improve quality by stopping separation can reduce quality everywhere else in the can.
Who decides the process, and it is not the ingredient supplier
This has to be said plainly.
Thermal process schedules are established and validated by a qualified thermal processing authority, for a specific product in a specific container in a specific retort. That is a regulated responsibility and an ingredient supplier does not hold it. Nothing in this note is a process recommendation and no time or temperature appears in it for that reason.
What this note is for is the other half of the same problem. A formulation change can change how the can heats, and the people holding the schedule cannot know that unless they are told. A thickener changed, a starch swapped for one that swells at a different temperature, a particulate size altered, a fill weight moved: any of those can shift a product from one heating behaviour to another. The failure mode worth avoiding is a recipe change that goes through as a recipe change and never reaches the person who validated the process.
The heating curve that breaks in the middle
What this makes the job
Treat viscosity as a process variable, not only a texture one. It is one of several formulation properties that can affect heat penetration and the required process.
Say how the product heats before choosing how to thicken it. Convection, conduction, or convection then conduction. That decides which materials are even candidates.
Choose the starch for its swelling behaviour as well as its finished texture. Two starches that give the same viscosity in the finished product can give very different heating curves on the way there.
Take separation apart before thickening it away. Separation is often an emulsion or a suspension problem rather than a viscosity problem, and solving it without adding body avoids the whole trade described here.
And tell whoever holds the process schedule. Every time.
If a canned product is separating, or arriving softer than it should, tell us what is in it and what it is processed in, and we will look at both halves.
